The invention pertains to plasma physics, and in particular to and transportation and use of positively charged particle beams.
Particle beams of positively charged atoms have potentially a number of useful applications, among which are thin film deposition, semiconductor doping, use as a general laboratory tool, a source of pulsed neutrons (i.e. by impacting a suitable target), shock hardening of materials, advanced military weaponry, and, most interestingly, as the trigger in a nuclear fusion reactor. Unfortunately, a number of these applications require that after the beams are created in particle accelerators, they must travel a considerable distance to their targets. Because the beam is charged, and because the beam's atoms have significant thermal energy, the beam tends to dissipate unless steps are taken to hold it together. Several ways exist to do this, classified broadly as external and internal confinement approaches.
External confinement uses external magnetic or electrostatic lenses to focus the beam. An example is ballistic focusing, in which a lens focuses the beam at one point in space, beyond which it again expands; thus to maintain the beam intact over an appreciable distance requires a series of lens elements distributed along the path between the particle accelerator which creates the beam, and the beam's target, to repeatedly refocus the beam.
External confinement has several inherent drawbacks. Because the velocity of atoms in the beam is not uniform, a single lens used for ballistic transport tends to focus different atoms of different velocities at different spots, causing a net result akin to a blurred optical image. Therefore, the use of ballistic focusing imposes severe constraints on longitudinal beam dispersion, which are difficult to satisfy in accelerators. Furthermore, ballistic transport can only be used over limited distances.
Use of external confinement to transport an ion beam over long distances, or maintaining the beam at a small radius requires a sequence of lens elements. Such a large number of lenses requires a substantial investment in hardware and money. Lensing of this kind cannot be used in a physically hostile environment because the lensing hardware would be destroyed. This poses a particularly difficult problem if one wishes to use ion beams in a nuclear fusion reaction chamber. The chamber will contain high radiation levels, and extremely hot fragments from fusion reactions which would rapidly destroy any lensing apparatus, or for that matter any workers tasked with replacing the apparatus.
Internal confinement consists of providing a medium which sufficiently neutralizes the beam space charge, and insuring that an axial current flows of sufficient magnitude that it creates an azimuthal magnetic flux which confines the beam and prevents it from diverging, i.e. "pinches" the beam. A relativistic electron beam injected into an initially neutral gas will ionize the gas and generate electromagnetic fields which self-pinch the beam. This has been demonstrated in both high density gases (resistive regime) and low density gases (ion-focused regime). Ion-focused regime propagation is also possible using preionized plasma channels. Such channels can be generated by a laser or a low energy electron beam in any of a number of known ways.
Internal confinement, i.e. pinching, has none of the hardware drawbacks of external confinement. Unfortunately, it is widely believed that positive ion beams cannot self-pinch in this manner. Calculations indicate that relativistic ion beams propagating into a neutral gas in the resistive regime would ionize a plasma channel so quickly and so extensively that plasma currents would largely cancel the magnetic pinching flux which the ion beam generates on itself. The ion-focused regime mechanism, in which the channel electrons are expelled and a relativistic electron beam is pinched by the resulting positive ion channel, cannot work for a positive ion beam. Thus for ion beams, attempts at self-pinching appear self-defeating.
A variation of internal confinement is to generate a current-carrying guided electric discharge to provide the pinching magnetic field. In this method, a laser partially ionizes the gas along the desired path for the beam. A high voltage is applied at opposite ends of the ionization path. This causes further ionization by avalanche breakdown and induces a strong axial current which pinches the beam. Laser-guided discharges are cumbersome for many applications and cannot be used in many parameter regimes of interest.
A variation on internal confinement, counter-propagation using a relativistic electron beam, can in theory provided pinched propagation of an ion beam. In counter-propagation a self-pinched relativistic electron beam and a positive ion beam are propagated along the same axis but in opposite directions, i.e. through one another. Being oppositely directed and of opposite charge, each beam sets up reinforcing pinching fluxes which, together, could hold the ion beam pinched.
Accelerators to generate the required electron beams are large and expensive. Also, counter-propagation suffers the practical disadvantage that the ion beam's target will obstruct the oppositely directed electron beam. One could try circumventing this by using strong bending magnets to curve the electron beam in front of the target and into the beam. But this is technically difficult, and would require expensive magnets to be placed in a physically hostile environment, the major drawback of external confinement.